Guest Blog: Len Pole on the Erratic boulder
The stone which is now on a
concrete plinth outside the Museum was uncovered during excavations for the
extension to Acrows Engineering works in Ashdon Road in Autumn 1973. It was
presented to the Museum by Acrows and has aroused a good deal of curiosity and
interest since. The measures about 84” by 72”, has a well-rounded profile and
is about 18” at the centre. Its weight has been estimated at about three tons.
It is a glacial erratic
boulder, a piece of rock which has been transported from its original location
by the action of ice sheets, during a period from 500,000 to 100,000 years ago.
In this area the glaciers are thought to have moved on a broadly north-south
axis, entering the trumpet-shaped gap in the chalk escarpment and moving
southward along the valley of the Cam.
Evidence of its contact with the ice sheet can be had from the scratch
marks on the surface, especially near the rounded edge.
The boulder, which is of
limestone, was formed during the Upper Jurassic Period, that is, about 150
million years ago. These limestone concretions form wherever sufficient lime is
present to cement part of a stratum without there being enough to form a
continuous bed. There are at least three fossil impressions of ammonites on the
surface, as well as fossils of other Jurassic creatures.
Boulders such as this are
often riddled with internal divisions caused by the continuous application of
pressure over long periods of time. The cracks become infilled with a saturated
lime solution which crystallises into a kind of quartz which is harder than the
limestone itself. In some cases, these
crystalline plates or septa emerge on the surface of the stone, especially if
it is subjected to weathering. This septarian structure can be seen in other
boulders in the museum. The septa cannot be seen on the large stone, but it is
likely to possess this property internally.
Although these boulders are
common enough in this part of the country, they are rarely so large or so
well-preserved. Its occurrence in Walden illustrates the transporting power of
ice in the glacial period.
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